1. Field of the Invention
This invention relates to reflector device for large-size reflection telescopes, and more particularly to shape control of such reflectors.
2. Description of the Prior Art
Schematically shown in FIG. 1 is a conventional reflector, in which the reference 1 denotes boules which are employed as a unit material in the fabrication of reflectors. A plural number of boules 1 are overlapped one on another and bonded together to make a stack 2, and a plural number of such stacks 2 are arranged to form a surface of a concave shape, for example, a parabolic or hyperbolic surface.
In operation, the surface of the reflector 3, which has undergone polishing of extremely high precision to provide a reflecting surface of a predetermined concave shape such as parabolic or hyperbolic shape, reflects electromagnetic waves from a celestial body such as visible light or infrared rays to form an image of the celestial body at its focal point.
In this instance, since the coefficient of linear expansion of each boule 1 is not zero, the reflector 3 is subject to thermal deformation under varying temperature conditions. If the reflector 3 is uniform in coefficient of linear expansion over its entire body, it undergoes thermal deformation into a similar shape, converging the electromagnetic waves from a celestial body toward a position of similarity with respect to the original focal point to form an image of the celestial body at that position. However, as a matter of fact the reflector 3 inhomogeneity in coefficient of linear expansion, so that the surface of the reflector 3 is distorted by thermal deformations attributable to such inhomogeneity, resulting in deteriorations in image-forming performance quality.
Therefore, it has been the conventional practice to measure the thickness-wise distribution of the coefficient of linear expansion of each boule 1 to determine its variation rate beforehand and to combine the boules according to the data of variation rate in such a manner as to put the mean variation rate close to zero. Namely, each stack 2 is formed of a combination of boules 1 which are considered to counteract in thermal deformation, thereby to reduce the thermal deformation of the stack 2.
A number of such stacks 2 are arranged randomly to form the reflector 3.
The conventional reflector 3 with the above-described construction, however, has a problem that the reflector as a whole undergoes considerably large thermal deformation due to localized inhomogeneity in coefficient of linear expansion in the entire body of the reflector 3, which are caused by inhomogeneity among the stacks 2 in variation rate of the coefficient of linear expansion remaining in the thickness-wise direction or due to inhomogeneity among the stacks 2 in average coefficient of linear expansion.